AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2024: Question 5

✅ Correct answer

An ecosystem is the interaction of a community of living organisms and the non-living parts of their environment.

You could also say: the interaction of a community and its habitat.

💡 Key knowledge

• Biotic = living parts.
• Abiotic = non-living parts.
• An ecosystem includes both, and how they interact.

🧠 Exam technique

• Use the word interaction for full marks.
• Don’t just write “a habitat” or “a community” on its own — that is not complete enough.

❌ Common errors

• Only saying “a place where organisms live”.
• Leaving out the non-living/abiotic part.
• Not mentioning the relationship between the parts.

✅ Correct answers

Any two of the following:

• They may be predators, so they control the prey population.
• They may be prey, providing energy/food to another species.
• They may be a primary consumer, controlling plant or algae populations.
• They may alter the habitat for other species to survive.
• They may provide shelter for other species to survive.
• They may maintain biodiversity.
• They may disperse seeds or pollen.

💡 Key knowledge

• In ecology, species can affect population size and food webs.
• Otters are a good example of a predator that can help keep populations stable.
• Stable communities often have balanced feeding relationships.

🧠 Exam technique

• Write two separate points — one mark for each.
• Develop each point slightly: say how it helps stability.
• Named examples are allowed, but not needed.

❌ Common errors

• Writing vague statements like “they keep the ecosystem healthy” without explaining why.
• Repeating the same idea twice in different words.
• Saying only “they eat other animals” without linking to stability.

📐 Calculation: step-by-step

Data:

• Bacteria at 5.0 km = 216 (× 1000/mm³)
• Bacteria where sewage was added = 4 (× 1000/mm³)

1. Write the ratio: 216 ÷ 4
2. Calculate: 216 ÷ 4 = 54
3. State the answer clearly: 54 times greater

✅ Correct answer

54

So the number of bacteria at 5.0 km was 54 times greater.

🧠 Exam technique

• Use the numbers exactly as given in the table.
• This is a ratio question, so divide the larger number by the smaller number.
• Because the question asks “how many times greater”, a plain number is enough.

❌ Common errors

• Adding the numbers instead of dividing.
• Using the wrong row from the table.
• Forgetting to answer in words: “times greater”.

✅ Correct answer

As the number of bacteria increases, the concentration of dissolved oxygen decreases.

The relationship is not proportional because the two variables do not increase in the same ratio.

💡 Key knowledge

• The table shows a negative correlation: more bacteria, less oxygen.
• At 0.0 km there are 4 bacteria and 5.4 mg dm⁻³ oxygen.
• At 5.0 km there are 216 bacteria and 2.7 mg dm⁻³ oxygen.
• These values show the numbers change in opposite directions, not in a fixed ratio.

🧠 Exam technique

• Use the table to support your answer.
• To show something is not proportional, say the two quantities do not increase/decrease in the same ratio.
• A strong answer compares at least two points from the table.

❌ Common errors

• Saying “they are inversely proportional” — this was not the wording the mark scheme wanted.
• Writing only “bacteria increases and oxygen decreases” without saying why this disproves proportionality.
• Confusing “correlation” with “proportional”.

✅ Correct answer

1. Decreased oxygen means fewer fish survive because fish need oxygen for aerobic respiration.

2. Otters then have less food because they eat mainly fish.

3. Therefore, the otter population decreases (or otters may move to another habitat).

💡 Key knowledge

• Fish need dissolved oxygen to respire aerobically.
• If oxygen levels fall, fish may die or become fewer in number.
• Otters depend on fish as a food source, so fewer fish means less food for otters.
• Less food usually leads to fewer otters surviving or reproducing.

🧠 Exam technique

• Use a chain of reasoning: oxygen ↓ → fish ↓ → food for otters ↓ → otter population ↓.
• There are 3 marks, so give three linked ideas.
• You can also mention that more bacteria may be present, leading to more fish dying from infection.

❌ Common errors

• Saying otters “need oxygen from the water” — the mark scheme does not accept that.
• Talking only about bacteria and forgetting the fish–otter link.
• Stopping at “fish die” without explaining the effect on otters.
• Saying “energy from respiration decreases” — not acceptable here.

💡 Top-level features

• Used precise biology language: biotic, abiotic, predator, prey, proportional.
• Read the table carefully and used the data in explanations.
• Wrote cause-and-effect chains rather than isolated facts.
• Gave just enough detail for the mark scheme, without overcomplicating it.

🧠 Final exam tips

• For definition questions, learn the exact wording.
• For “explain” questions, always link the steps together.
• For data questions, quote numbers from the table.
• Check whether the question wants a trend, a calculation, or a reason.